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va_start with C23Section: 17.14.2 [cstdarg.syn] Status: New Submitter: Jakub Jelinek Opened: 2025-10-01 Last modified: 2025-10-03
Priority: Not Prioritized
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Discussion:
P3348R4 changed the va_start macro to match C23,
but the following wording from C is not present in C++:
If any additional arguments expand to include unbalanced parentheses, or a preprocessing token that does not convert to a token, the behavior is undefined.
The importance of that wording was not realized during review of P3348R4.
The wording is intended to ensure that any discarded arguments to
va_start are actually lexable by the compiler,
rather than containing unbalanced parentheses or brackets.
It also makes the following undefined:
#define BAD ); format_disk(
va_start(ap, BAD);
Proposed resolution:
This wording is relative to N5014.
Modify 17.14.2 [cstdarg.syn] as indicated:
(1.2) — If more than one argument is present for
va_startand any of the second or subsequent arguments expands to include unbalanced parentheses, or a preprocessing token that does not convert to a token, the program is ill-formed, no diagnostic required. The preprocessing tokens comprising the second and subsequent arguments tova_start(if any) are discarded. [Note 1:va_startaccepts a second argument for compatibility with prior revisions of C++. — end note]